1. Field of the Invention
The present invention relates to a liquid-crystal display device employing a liquid-crystal panel and, more particularly, to a liquid-crystal driving circuit and liquid-crystal driving method for improving the response speed of the liquid crystal.
2. Description of the Related Art
Liquid crystals have the drawback of being unable to respond to rapidly changing moving pictures, because their transmissivity changes according to a cumulative response effect. One method of solving this problem is to improve the response speed of the liquid crystal by increasing the liquid-crystal driving voltage above the normal driving voltage when the gray level changes.
FIG. 72 shows an example of a liquid-crystal driving device that drives a liquid crystal by the above method; details are given in, for example, Japanese Unexamined Patent Application Publication No. 6-189232. Reference numeral 100 in FIG. 72 denotes an A/D conversion circuit, 101 denotes an image memory storing the data for one frame of a picture signal, 102 denotes a comparison circuit that compares the present image data with the image data one frame before and outputs a gray-level change signal, 103 denotes the driving circuit of a liquid-crystal panel, and 104 denotes the liquid-crystal panel.
Next, the operation will be described. The A/D conversion circuit 100 samples the picture signal on a clock having a certain frequency, converts the picture signal to image data in digital form, and outputs the data to the image memory 101 and comparison circuit 102. The image memory 101 delays the input image data by an interval equivalent to one frame of the picture signal, and outputs the delayed data to the comparison circuit 102. The comparison circuit 102 compares the present image data output by the A/D conversion circuit 100 with the image data one frame before output by the image memory 101, and outputs a gray-level change signal, indicating changes in gray level between the two images, to the driving circuit 103, together with the present image data. The driving circuit 103 drives the display pixels of the liquid-crystal panel 104, supplying a higher driving voltage than the normal liquid-crystal driving voltage for pixels in which the gray level has increased, and a lower voltage for pixels in which the gray level has decreased, according to the gray-level change signal.
A problem in the image display device shown in FIG. 72 is that as the number of pixels displayed by the liquid-crystal panel 104 increases, so does the amount of image data written into the image memory 101 for one frame, so the necessary memory size increases. In the image display device described in Japanese Unexamined Patent Application Publication No. 4-204593, one address in the image memory is assigned to four pixels, as shown in FIG. 73, to reduce the size of the image memory 101. The size of the image memory is reduced because the pixel data stored in the image memory are decimated, excluding every other pixel horizontally and vertically; when the image memory is read, the same image data are read for the excluded pixels as for the stored pixel, several times. For example, the data at address 0 are read for pixels (a, B), (b, A), and (b, B).
As described above, the response speed of the liquid crystal can be improved by increasing the liquid-crystal driving voltage above the normal liquid-crystal driving voltage when the gray level changes from the gray level one frame before. Since the liquid-crystal driving voltage is increased or reduced, however, only according to changes in the magnitude relationship between the gray levels, if the gray level increases from the gray level one frame before, the same higher driving voltage than the normal voltage is applied regardless of the size of the increase. Therefore, when the gray level changes only slightly, an overly high voltage is applied to the liquid crystal, causing a degradation of image quality.
If the size of the image memory 101 is reduced by decimation of the image data in the image memory 101 as shown in FIG. 73, the problem described below occurs. FIGS. 74A to 74D illustrate the problem caused by decimation. FIG. 74A shows image data for frame n+1, FIG. 74B shows image data for the image in frame n+1 shown in FIG. 74A after decimation, FIG. 74C shows the image data read by interpolation of the decimated pixel data, and FIG. 74D shows the image data for frame n, one frame before. The image for frame n and the image for frame n+1 are identical, as shown in FIGS. 74A and 74D.
If decimation is carried out as shown in FIG. 74C, the pixel data at (A, a) are read as the pixel data for (B, a) and (B, b), and the pixel data at (A, c) are read as the pixel data for (B, c) and (B, d). Thus pixel data with gray level 50 are read as pixel data for a gray level that is actually 150. Therefore, even though the image has not changed from the frame before, pixels (B, a), (B, b), (B, c), and (B, d) in frame n+1 are driven with a higher driving voltage than the normal voltage.
Thus when decimation is carried out, the voltages for the pixels with decimated pixel data are not controlled accurately, and the image quality is degraded by the application of unnecessary voltages.